1. Field of the Invention
The present invention relates to an image forming apparatus according to an electrophotographic process.
2. Description of the Related Art
Image formation according to the electrophotographic process is widely employed in copying machines, printers, facsimile apparatuses and so on. In image formation according to the electrophotographic process, a photoreceptor having a photosensitive layer which is formed on a surface thereof and contains a photoconductive substance is used. After the surface of the photoreceptor is uniformly charged by giving electric charges, an electrostatic latent image corresponding to image information is formed through various image forming processes, the electrostatic latent image is developed with toner supplied from a developing device to be a toner image, and the resultant toner image is transferred and fixed to a recording medium such as a recording sheet.
In order to improve the quality of an image transferred and fixed to a recording medium, and improve transfer efficiency that is a ratio of the amount of toner transferred onto a recording medium to the amount of toner of a toner image formed on a photoreceptor, various transfer methods and fixing methods have been proposed up to now.
As an example of such transfer methods can be given a transfer method comprising the steps of: transferring a toner image formed on a photoreceptor onto a primary transfer medium; transferring the toner image transferred to the primary transfer medium to a secondary transfer medium having a heat source inside; and transferring and fixing the toner image transferred to the secondary transfer medium to a recording sheet that is a recording medium (refer to Japanese Unexamined Patent Publication JP-A 63-34572 (1988).
According to the technique disclosed in JP-A 63-34572, it is possible, by preheating toner on the secondary transfer medium, to sufficiently heat the toner before transferring to a recording medium. According to the technique, by making the temperature of the toner reach the neighborhood of a critical interface temperature on the secondary transfer medium, and then transferring and fixing the toner to a recording medium, namely, a recording sheet at a fixing nip area, it is possible to secure a fix level at a relatively low fixing temperature even in a high-speed system machine. Moreover, according to the technique, adoption of a non-electric field transfer system (a thermal transfer system using heat) makes it possible to increase transfer efficiency at the time of transferring a toner image from the primary transfer medium to the secondary transfer medium as compared with a case where a conventional electric field transfer system is adopted, and makes it possible to obtain an image of higher quality with less toner scattering.
However, the technique disclosed in JP-A 63-34572 has a problem as described below. In the technique disclosed in JP-A 63-34572, a silicone RTV (room temperature vulcanizable) belt having good surface smoothness is used as the primary transfer medium, and a heating roller having a diameter of 120 mm provided with silicone RTV on an aluminum (Al) core metal having a halogen lamp inside is used as the secondary transfer medium.
In the technique disclosed in JP-A 63-34572, toner is transferred from the photoreceptor belt to the primary transfer medium, from the primary transfer medium to the secondary transfer medium, and from the secondary transfer medium to a recording sheet. In a case where silicone RTV is used as a material of a surface layer of the transfer medium, when toner is transferred from the primary transfer medium to the secondary transfer medium by thermal transfer, the toner is likely to stick and remain on the silicone RTV surface of the primary transfer medium, and division of toner, namely, toner offset occurs at a nip area between the photoreceptor belt and the primary transfer medium, because toner releasability of silicone RTV is insufficient.
In a case where division of toner, namely, toner offset occurs, part of the toner remains on the primary transfer medium, and toner for forming a toner image later gets short, with the result that there arises a problem such that image quality decreases. Moreover, also when a toner image is transferred from the secondary transfer medium to a recording sheet, because of the insufficient releasability, toner offset occurs, and toner remains on the secondary transfer medium, with the result that there arises a problem such that image quality significantly decreases.
To solve such a problem of decrease in image quality due to toner offset in an ordinary fixing system, it is necessary to apply a liquid having releasability such as silicone oil to the silicone RTV in order to increase toner releasability. However, in a so-called two-stage transfusing system as disclosed in JP-A 63-34572 that thermally transfers from the photoreceptor belt to the primary transfer medium and then from the primary transfer medium to the secondary transfer medium and thereafter transfers and fixes to a recording sheet, there is an adverse effect by application of the silicone oil, that is a problem of oil contamination of the photoreceptor belt and a developing portion with the silicone oil.
The cause of the above is that due to contact of the primary transfer belt and the photoreceptor belt with each other, the oil reaches the photoreceptor belt from the primary transfer medium and adversely affects formation of an electrostatic latent image and formation of a toner image at the time of development. Further, since the oil also reaches the inside of a development tank through the photoreceptor belt, there is also a problem such that a failure in development is induced.
There is another adverse effect such that due to existence of the silicone oil having good releasability at a nip area between the primary transfer medium and the secondary transfer medium, the secondary transfer medium acquires high releasability from the silicone oil, and thermal transfer of toner from the primary transfer medium to the secondary transfer medium in a good condition is impossible. That is to say, there is a problem such that a thermal transfer property is significantly impaired.
In the technique disclosed in JP-A 63-34572, in order to avoid the problems resulting from use of the oil, a release agent that gives releasability is added into a developer. By adding a release agent into a developer, it is possible to realize an oilless system that allows transferring and fixing without applying silicone oil to a transfer medium and a fixing roller. However, in the case of adopting the oilless system, it is usual to form the surfaces of the transfer medium and the fixing roller with tubes made of a copolymer of tetrafluoroethylene and perfluoroalkyl vinyl ether (abbreviated to PFA) or coat the surfaces with PFA. In the case of using a developer to which a release agent is added and obtaining desired releasability, it is actually difficult to use a surface material other than PFA.
Since the silicone RTV illustrated as an example of the material of the surface layers of the primary and secondary transfer mediums in the technique disclosed in JP-A 63-34572 has poor releasability, it is presumed that decrease in image quality due to the poor releasability occurs on the primary transfer medium or on the secondary transfer medium even if a developer into which a release agent is added is used.
Further, the technique disclosed in JP-A 63-34572 also has a problem as described below from the perspective of power saving.
At present, by the request for improvement of image formation efficiency, a color copier, a multifunction peripheral and so on are required to have a process speed of about 200 mm/sec to 400 mm/sec in general. However, generally, in the case of using the high-speed process as described above, the maximum power consumption far exceeds 1.5 kVA (100V, 15 A) that is an allowable limit of electric power of a commercial power source. Therefore, it becomes necessary to prepare a large-capacity power source when using a high-speed process machine in an ordinary office, home and so on.
Power consumption of a fixing portion forms the largest proportion in the details of power consumption of an image forming apparatus, and it is necessary to set a fixing temperature high in order to prevent occurrence of toner offset when continuously printing a plurality of sheets at a high speed, with the result that the power consumption tends to further increase.
Further, since the number of sheets of image formation per unit time is large in the case of the high-speed process, heat of the surface of the fixing roller is removed by recording sheets while the recording sheets are continuously passing, the temperature of the surface of the fixing roller rapidly decreases, and it takes long hours before the temperature returns to a predetermined fixing temperature. In short, there is a problem of so-called decrease of a heat follow-up property.
A conventionally used fixing system is a system in which after a toner image is transferred onto a recording sheet by the electrostatic action in a transfer portion, a recording sheet is conveyed to a nip area formed between a pair of heated rollers (a roller pair that is composed of a fixing roller having a halogen lamp inside to heat and a pressure roller pressing and contacting the fixing roller, and that forms a nip between the rollers), and toner is melted and fixed to the recording sheet at the nip area. In this system, increase of the heat follow-up property against decrease of the fixing temperature while sheets are continuously passing is attempted by making a rubber layer of the surface of the fixing roller thin, or selecting a lamp of high wattage as the heating lamp installed in the fixing roller.
Accordingly, in the high-speed process machine, in order to sufficiently fix while satisfying an offset property, a heat source of high wattage is installed, heating efficiency is increased by making a diameter of the fixing roller large, and a fixing nip width is secured by winding an endless belt serving as a pressure member on the fixing roller so as to abut.
The technique disclosed in JP-A 63-34572 adopts a method of using the heating roller having a large diameter of 120 mm as the secondary transfer medium that transfers and fixes transferred toner to a transfer sheet. In a case where the heating roller has a large diameter of 120 mm, a space between the halogen lamp and the Al core metal serving as a roller core metal is too large, and hence, heat transfer efficiency is bad, and it takes much time to increase the temperature. Further, since a surface area of the secondary transfer medium is large, the amount of heat radiated from the secondary transfer medium into the air is large, and heating loss is large. Accordingly, use of a large-diameter roller as in the technique disclosed in JP-A 63-34572 runs counter to energy saving because loss of electric power increases, and causes a problem such that the aim of adoption of a transfuse system to fix at a high speed with small power consumption is not fully achieved.
Further, since a time required for transfer of a toner image is short in the high-speed process, transfer efficiency is as low as 85% to 95%, the efficiency of use of toner is bad, and the amount of waste toner is large, which leads to a lot of waste. Therefore, in the high-speed process, it is desired to increase transfer efficiency and decrease waste toner production.